/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

import java.io.Serializable;
import java.math.BigInteger;
import java.util.NoSuchElementException;

/**
 * A descriptor for a <i>discrete</i> {@code Comparable} domain such as all
 * {@link Integer} instances. A discrete domain is one that supports the three basic
 * operations: {@link #next}, {@link #previous} and {@link #distance}, according
 * to their specifications. The methods {@link #minValue} and {@link #maxValue}
 * should also be overridden for bounded types.
 *
 * <p>A discrete domain always represents the <i>entire</i> set of values of its
 * type; it cannot represent partial domains such as "prime integers" or
 * "strings of length 5."
 *
 * <p>See the Guava User Guide section on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/RangesExplained#Discrete_Domains">
 * {@code DiscreteDomain}</a>.
 *
 * @author Kevin Bourrillion
 * @since 10.0
 */
@GwtCompatible
@Beta
public abstract class DiscreteDomain<C extends Comparable> {

  /**
   * Returns the discrete domain for values of type {@code Integer}.
   * 
   * @since 14.0 (since 10.0 as {@code DiscreteDomains.integers()})
   */
  public static DiscreteDomain<Integer> integers() {
    return IntegerDomain.INSTANCE;
  }

  private static final class IntegerDomain extends DiscreteDomain<Integer>
      implements Serializable {
    private static final IntegerDomain INSTANCE = new IntegerDomain();

    @Override public Integer next(Integer value) {
      int i = value;
      return (i == Integer.MAX_VALUE) ? null : i + 1;
    }

    @Override public Integer previous(Integer value) {
      int i = value;
      return (i == Integer.MIN_VALUE) ? null : i - 1;
    }

    @Override public long distance(Integer start, Integer end) {
      return (long) end - start;
    }

    @Override public Integer minValue() {
      return Integer.MIN_VALUE;
    }

    @Override public Integer maxValue() {
      return Integer.MAX_VALUE;
    }

    private Object readResolve() {
      return INSTANCE;
    }

    @Override
    public String toString() {
      return "DiscreteDomains.integers()";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns the discrete domain for values of type {@code Long}.
   * 
   * @since 14.0 (since 10.0 as {@code DiscreteDomains.longs()})
   */
  public static DiscreteDomain<Long> longs() {
    return LongDomain.INSTANCE;
  }

  private static final class LongDomain extends DiscreteDomain<Long>
      implements Serializable {
    private static final LongDomain INSTANCE = new LongDomain();

    @Override public Long next(Long value) {
      long l = value;
      return (l == Long.MAX_VALUE) ? null : l + 1;
    }

    @Override public Long previous(Long value) {
      long l = value;
      return (l == Long.MIN_VALUE) ? null : l - 1;
    }

    @Override public long distance(Long start, Long end) {
      long result = end - start;
      if (end > start && result < 0) { // overflow
        return Long.MAX_VALUE;
      }
      if (end < start && result > 0) { // underflow
        return Long.MIN_VALUE;
      }
      return result;
    }

    @Override public Long minValue() {
      return Long.MIN_VALUE;
    }

    @Override public Long maxValue() {
      return Long.MAX_VALUE;
    }

    private Object readResolve() {
      return INSTANCE;
    }

    @Override
    public String toString() {
      return "DiscreteDomains.longs()";
    }

    private static final long serialVersionUID = 0;
  }
  
  /**
   * Returns the discrete domain for values of type {@code BigInteger}.
   */
  // TODO(kevinb): make sure it's tested, and make it public
  static DiscreteDomain<BigInteger> bigIntegers() {
    return BigIntegerDomain.INSTANCE;
  }

  private static final class BigIntegerDomain extends DiscreteDomain<BigInteger>
      implements Serializable {
    private static final BigIntegerDomain INSTANCE = new BigIntegerDomain();

    private static final BigInteger MIN_LONG =
        BigInteger.valueOf(Long.MIN_VALUE);
    private static final BigInteger MAX_LONG =
        BigInteger.valueOf(Long.MAX_VALUE);

    @Override public BigInteger next(BigInteger value) {
      return value.add(BigInteger.ONE);
    }

    @Override public BigInteger previous(BigInteger value) {
      return value.subtract(BigInteger.ONE);
    }

    @Override public long distance(BigInteger start, BigInteger end) {
      return end.subtract(start).max(MIN_LONG).min(MAX_LONG).longValue();
    }

    private Object readResolve() {
      return INSTANCE;
    }

    @Override
    public String toString() {
      return "DiscreteDomains.bigIntegers()";
    }

    private static final long serialVersionUID = 0;
  }
  
  /** Constructor for use by subclasses. */
  protected DiscreteDomain() {}

  /**
   * Returns the unique least value of type {@code C} that is greater than
   * {@code value}, or {@code null} if none exists. Inverse operation to {@link
   * #previous}.
   *
   * @param value any value of type {@code C}
   * @return the least value greater than {@code value}, or {@code null} if
   *     {@code value} is {@code maxValue()}
   */
  public abstract C next(C value);

  /**
   * Returns the unique greatest value of type {@code C} that is less than
   * {@code value}, or {@code null} if none exists. Inverse operation to {@link
   * #next}.
   *
   * @param value any value of type {@code C}
   * @return the greatest value less than {@code value}, or {@code null} if
   *     {@code value} is {@code minValue()}
   */
  public abstract C previous(C value);

  /**
   * Returns a signed value indicating how many nested invocations of {@link
   * #next} (if positive) or {@link #previous} (if negative) are needed to reach
   * {@code end} starting from {@code start}. For example, if {@code end =
   * next(next(next(start)))}, then {@code distance(start, end) == 3} and {@code
   * distance(end, start) == -3}. As well, {@code distance(a, a)} is always
   * zero.
   *
   * <p>Note that this function is necessarily well-defined for any discrete
   * type.
   *
   * @return the distance as described above, or {@link Long#MIN_VALUE} or
   *     {@link Long#MAX_VALUE} if the distance is too small or too large,
   *     respectively.
   */
  public abstract long distance(C start, C end);

  /**
   * Returns the minimum value of type {@code C}, if it has one. The minimum
   * value is the unique value for which {@link Comparable#compareTo(Object)}
   * never returns a positive value for any input of type {@code C}.
   *
   * <p>The default implementation throws {@code NoSuchElementException}.
   *
   * @return the minimum value of type {@code C}; never null
   * @throws NoSuchElementException if the type has no (practical) minimum
   *     value; for example, {@link java.math.BigInteger}
   */
  public C minValue() {
    throw new NoSuchElementException();
  }

  /**
   * Returns the maximum value of type {@code C}, if it has one. The maximum
   * value is the unique value for which {@link Comparable#compareTo(Object)}
   * never returns a negative value for any input of type {@code C}.
   *
   * <p>The default implementation throws {@code NoSuchElementException}.
   *
   * @return the maximum value of type {@code C}; never null
   * @throws NoSuchElementException if the type has no (practical) maximum
   *     value; for example, {@link java.math.BigInteger}
   */
  public C maxValue() {
    throw new NoSuchElementException();
  }
  
}